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Fostex FE-108 Σ

Pair of Fostex FE-108 Sigma 4 inch Fullrange Drivers

FE-108 Σ is the smallest model of the legendary Sigma series. Unlike the bigger models it has half-roll surround and a dust cap instead of a whizzer cone.

This is the manufacturer's brochure  Link To Fostex FE-108 Sigma Manufacturer's Datasheet

My Measurements

Driver Properties Thiele - Small Parameters
Chassis material Vacuum Molded Aluminum Alloy   Parameter Measured Datasheet
Membrane material Banana Pulp Paper Fs 70.5Hz 80 Hz
Magnet System Ferrite Re 6.5 Ω 6.5 Ω
Surround Cloth Qms 2.10 2.21
Voice Coil Diameter 26 mm Qes 0.21 0.22
Nominal Impedance 8 Ω Qts 0.19 0.20
Sensitivity 92 dB/W/m   Vas 10.3 ltr 7.07 ltr
Rated Power 5W Le 0.25 mH N/A

SPL Responce of Fostex FE-108 Sigma at 1m on axis and 15 30 deg off axis

Frequency response at 1m with 2.83 V input RED - on axis, BLUE - 15 º off axis, GREEN - 30 º off axis

Second, Third and Fourth Harmonic of Fostex FE-108 Sigma in 18 ltr sealed box

The second, third and fourth harmonic curves measured at 2.83V input RMS.

Burst decay Responce of Fostex FE-108 Sigma at 1m in 18 ltr sealed box

Burst decay response, measured at 1m on axis

The impedance of Fostex FE-108 Sigma in 18 ltr seled box

The impedance curve in 18 ltr sealed box


This driver has a typical T/S parameters for a backloaded horn applications. It keeps the excellent sound signature of the Sigma series with surprising solid sound performance for its dimensions. 

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